The invention relates generally to the field of Raman spectroscopy. More specifically, the invention relates to a handheld fiber optic probe that uses a thermoelectric cooler (TEC) to condense volatile organic compounds (VOCs) onto a SERS substrate for real-time monitoring and in-situ monitoring of VOCs in gas, liquid, and soil environments.
Raman spectroscopy is an emission technique that involves inelastic scattering of incident laser energy and results in spectral peaks that are frequently shifted from the incident energy. The Raman bands arise from changes in polarizability in a molecule during vibration. As a result, virtually all organic molecules display a characteristic Raman emission. Therefore, a Raman-based sensor would not be limited to a specific class of molecules as is the case for the laser induced fluorescence (LIF) sensor. FIG. 1 shows Raman spectra obtained for carbon tetrachloride (CCl4), chloroform (CHCl3), and methylene chloride (CH2Cl2). These chlorinated solvents vary in the number of hydrogen and chlorine atoms. Yet they can easily be distinguished by their Raman spectra. Unlike fluorescence, the Raman peaks are very narrow. The inherently high resolution of Raman spectra often permits the analysis and identification of several components in a mixture simultaneously.
Despite the advantages of Raman spectroscopy over other spectroscopic techniques, Raman spectroscopy is, inherently, an insensitive technique. In the 1970s, it was discovered that Raman scattering from molecules adsorbed on such noble metals as silver, copper, and gold can be enhanced by as much as 106 to 107. The phenomenon, surface-enhanced Raman spectroscopy (SERS), has been the subject of intensive theoretical and experimental research. More than one mechanism is involved in the SERS phenomenon. Initially, the SERS technique was used as a means to probe adsorption at metal interfaces both in electrochemical and gas-phase environments. This technique has proven useful in deducing the effects of interfacial structure and reactivity on the adsorption process. However, the sensitivity of the technique as well as its exceptional spectral selectivity has made SERS attractive for a broad range of analytical applications. SERS can be used for trace organic analysis and as a detection method in gas chromatography, liquid chromatography, and thin layer chromatography. Electrochemical SERS and SERS of chemically modified surfaces have been used to detect aromatic compounds and chlorinated hydrocarbons, organic contaminants of environmental concern, in the ppm concentration range.
Although SERS is very sensitive, the technique requires intimate contact between the SERS active surface and analyte. In turn, this requires that the analyte adsorbs to the SERS active surface. If SERS spectra need to be obtained in real time and in-situ, then the reaction between the SERS substrate and the analyte needs to be reversible.
A sensor design which would (1) be compact and robust and (2) extract VOC vapors from the air and concentrate them onto the SERS substrate would result in a sensor that could be used to monitor VOCs in real time and in-situ. Besides environmental monitoring, such a sensor could be used for homeland security and force protection. There is a real concern that terrorists could poison water supplies using readily available toxic industrial chemicals such as the BTEX compounds and chlorinated solvents.